The present invention relates to a microwave oscillation circuit, and more specifically to a microwave oscillation circuit using a dielectric resonator.
Referring to FIG. 2, there is shown a simplified circuit layout pattern diagram of a microwave oscillation circuit of the type which uses a dielectric resonator and which is used at a low gain at an operating frequency. The shown microwave oscillation circuit includes a dielectric resonator 53 mounted on a not-shown dielectric substrate, a pair of stubs 51 and 52 each formed on the not-shown dielectric substrate and located adjacent to the dielectric resonator 53 to be coupled with the dielectric resonator 53 at a resonation frequency, a bipolar transistor or a field effect transistor 54 located on the not-shown dielectric substrate and electrically connected to the pair of stubs 51 and 52, respectively, and an oscillation output terminal 56 connected to the stub 52. The transistor 54 is driven with DC voltages supplied from a biasing circuit 55 including bias resistors.
The dielectric resonator 53 is formed of a solid body of dielectric material having a large dielectric constant sufficient to operate as a resonator. The stub 52 and the dielectric resonator 53 constitute a microwave resonating circuit, and the stub 51 and the dielectric resonator 53 constitutes a feedback circuit for the transistor 54.
The oscillation circuit is constituted of the transistor 54, the stubs 51 and 52 and the dielectric resonator 53. An oscillation signal generated in the oscillation circuit thus formed is picked out from the oscillation output terminal 56. Therefore, the transistor 54, the stubs 51 and 52, the dielectric resonator 53, and the oscillation output terminal 56 constitutes a high frequency circuit (microwave circuit).
As mentioned above, the transistor 54 is formed of either a bipolar transistor or a field effect transistor. When the transistor 54 is formed of the bipolar transistor, an emitter terminal of the bipolar transistor is connected to ground. A base terminal and a collector terminal of the bipolar transistor are connected to the stub 51 and the stub 52, respectively. On the other hand, when the transistor 54 is formed of the field effect transistor, a source terminal of the field effect transistor is connected to ground. A gate terminal and a drain terminal of the field effect transistor are connected to the stub 51 and the stub 52, respectively. The biasing circuit 55 supplies necessary DC voltages to the base (or the gate) and the collector (or the drain) of the transistor 54, respectively, and therefore is constituted independently of the high frequency circuit as mentioned above.
For only convenience, the following description will be described under the assumption that the transistor 54 is formed of the bipolar transistor, but it should not be forgotten that as mentioned above, the transistor 54 can be formed of either a bipolar transistor or a field effect transistor.
In the above mentioned microwave oscillation circuit having the above mentioned construction, if the impedance of a load connected to the oscillation output terminal 56 varies, the impedance of the stub 52 correspondingly varies, with the result that the oscillation characteristics varies, and simultaneously, an operation stability at the side of the stub 51 becomes deteriorated. Specifically, when the impedance of the load varies, the oscillation output level becomes unstable, and in an extreme case, the oscillation stops.
Furthermore, since the oscillation circuit is of the feedback type, an isolation between the base and the collector of the transistor 54 (between the gate and the drain in the case of the field effect transistor) is not so good, so that variation on the output side is transferred to the base (or the gate), with the result that the unstability becomes large.
Accordingly, it is an object of the present invention to provide a microwave oscillation circuit using a dielectric resonator, which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide a microwave oscillation circuit using a dielectric resonator, capable of maintaining a stable oscillation independently of variation in a load impedance.
The above and other objects of the present invention are achieved in accordance with the present invention by a microwave oscillation circuit comprising a dielectric resonator located on a dielectric substrate, a first high frequency transmission line formed on the dielectric substrate and having one end coupled to the dielectric resonator at a resonation frequency, and a second high frequency transmission line formed on the dielectric substrate and having one end coupled to the dielectric resonator at the resonation frequency, and a transistor mounted on the dielectric substrate and having an input terminal connected to the other end of the first high frequency transmission line and an output terminal connected to the other end of the second high frequency transmission line, so that when the transistor is driven with a DC voltage, the transistor oscillates so that an oscillation output is outputted from the output terminal of the transistor, the microwave oscillation circuit further includes a bias resistor located in the neighborhood of a connection point between the other end of the first high frequency transmission line and the input terminal of the transistor, the bias resistor having a resistance which basically determines a bias voltage supplied to the input terminal of the transistor and which is enough to make high the impedance of a bias voltage supplying circuit including the bias resistor, viewed at the input terminal of the transistor.
With this arrangement, since the bias resistor for basically determining the bias voltage supplied to the input terminal of the transistor is located in the neighborhood of the connection point between the input terminal of the transistor and the other end of the first high frequency transmission line, although the microwave oscillation circuit behaves as a distributed parameter circuit, the impedance of the bias voltage supplying circuit viewed at the input terminal of the transistor becomes high by action of the resistance of the bias resistor. As the result, a high-frequency impedance of a microwave circuit at an input terminal side of the transistor is stabilized, and the amplitude modulation of the bias voltage supplied to the input terminal of the transistor, caused by the oscillation component, is avoided. Therefore, the oscillation stability is improved, and the oscillation output level are stabilized. Therefore, it is possible to completely avoid the stop of the oscillation caused by a variation in the impedance of the load.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.